Exhaust brake systems are widely fitted to compression ignition engines (diesel engines) used to power large vehicles such as trucks. Many types of exhaust brake system are known, all generally comprising a valve in the exhaust line from the engine which when activated at least substantially blocks the engine exhaust. This creates backpressure, which retards rotation of the engine providing a braking force, which is transmitted to the vehicle wheels through the vehicle drive train. With some exhaust brake systems the brake can be set to activate automatically when the engine throttle is closed (ie the driver lifts his foot from the throttle pedal). In other systems the exhaust brake may require manual activation by the driver, such as depression of a brake pedal. Throughout the description and claims, the term engine throttle is used. Although a diesel engine does not include a carburettor, which has an air throttle, the term is used to describe the liner, mechanism, or system that varies the rate at which fuel is delivered to a diesel engine and thus its power output. Therefore, whenever the term engine throttle is used, herein, it refers to any of these devices.
It is now conventional for large diesel engines to be turbocharged. A turbocharger essentially comprises an exhaust driven turbine wheel, which rotates a compressor wheel. The turbine wheel and compressor wheels are mounted in respective housings on opposite ends of a rotatable shaft which passes through a bore in a bearing housing located between the compressor and turbine housings. The bearing housing houses journal and thrust bearings and associated lubrication systems and seals.
The shaft is typically sealed with respect to the bore of the bearing housing at the turbine wheel end by a shaft seal ring (or piston ring) which sits in an annular groove provided in the shaft behind the turbine wheel. The shaft seal prevents oil from the bearing lubrication systems from leaking into the turbine housing which can cause blue smoke and oil drips from the exhaust pipe, as well as preventing exhaust gas pollution of the bearing housing which can cause overheating and adversely effect bearing life.
Problems can arise with this conventional shaft seal arrangement where the turbocharged engine has an engine brake valve located downstream of the turbine. As the engine brake is activated the backpressure in the exhaust line, and thus in the turbine wheel housing, rapidly rises and can reach 4-5 bar (4-5 times atmospheric pressure). As pressure behind the turbine wheel thus increases the shaft seal ring can be pushed inboard. This movement, together with the high rotational speed of the shaft, can generate excessive frictional heating, which can cause the shaft seal ring to overheat. This in turn can cause the ring to be more susceptible to movement in the bore and induce a reduction in life of the seal. This problem is particularly pronounced during the running in period of the engine/turbocharger before the seal ring “beds in”.